Add btn support for N=0 (disable newline insertion)

[Copilot]

btn currently requires N in [1, 32] and always inserts line breaks. This change extends the contract to allow N=0, which preserves whitespace stripping while disabling newline insertion entirely.

• Behavior update (N=0)

  • Updated C, Python, and Haskell implementations of btn to accept 0 as a valid argument.
  • N>0 behavior remains unchanged (chunked output with trailing newline handling).
  • N=0 now emits a single condensed stream with no inserted \n.

• Argument range and spec alignment

  • Updated range handling from [1, 32] to [0, 32] in btn docs/spec.
  • Clarified conditional newline rules in the specification for N>0 vs N=0.

• Targeted test coverage

  • Added a no-newline test case to test/.test/btn.test using argument 0.
  • Expected output validates fully condensed text with no trailing newline.

/* C behavior change */
if (num_char_until_newline > 0 &&
      printed_char_count == num_char_until_newline) {
   putchar('\n');
   printed_char_count = 0;
}
...
if (num_char_until_newline > 0 && printed_char_count != 0)
   putchar('\n');

[lshqqytiger]

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